1. Field of the Invention
The invention relates to a voltage measuring device for a high voltage alternating electric system, and more specifically, to a voltage measuring device of the capacitive voltage divider type for switchgear in which a voltage sensor is mounted integrally with the switchgear.
2. Description of the Prior Art
Generally, high voltage switchgear for alternating power systems include operating and control elements, such as motors, relays, and indicating lamps, which are connected between a low voltage AC or DC source and ground. When a capacitive type voltage sensor is integrally mounted to the high voltage switchgear, it is usually connected to a remote signal processing circuit located in a low voltage control compartment of the switchgear cubicle, or in the case of a pole mounted circuit interrupter, in a separate, low voltage control cabinet. Consequently, current flow through the switchgear operating and control elements can induce noise current in the conductors connecting the voltage sensor to the signal processing circuit and adversely affect the accuracy of the signal supplied to the signal processing circuit.
U.S. Pat. No. 3,970,932, to Harvey, discloses a hot line detection system for switchgear in which a high voltage conductor and a cylindrical electrode concentrically spaced about the high voltage conductor are embedded in an epoxy housing, which is mounted within a switchgear cubicle. The cylindrical electrode is connected through a ballast resistor and a neon lamp on the front panel of the switchgear cubicle to ground. When the high voltage conductor is energized, the cylindrical electrode is capacitively coupled to the energized line, to provide sufficient energy to light the neon lamp.
There is no discussion in U.S. Pat. No. 3,970,932, concerning any detrimental effect on the voltage signal supplied by the cylindrical electrode caused by current flow through the switchgear operating or control elements. Since this hot line detection device is only used to indicate whether or not a high voltage line is energized, the effect of any induced noise current may be of no practical significance.
U.S. Pat. No. 4,074,193, to Kohler, discloses a combined current and voltage measuring apparatus which includes a rod-shaped primary conductor and a cylindrical electrode spaced about the primary conductor, which are embedded in a cast resin body. The primary conductor and the cylindrical electrode form a first capacitor, with the portion of the cast resin between the primary conductor and the cylindrical electrode serving as the dielectric of the first capacitor. The cylindrical electrode is connected to ground through a second capacitor which is located outside of the cast resin housing. The voltage across the second capacitor, which is proportional to the voltage between the primary conductor and ground, is amplified and supplied to a voltmeter. There is no discussion in this patent concerning how the voltage measuring apparatus disclosed therein can be used with high voltage switchgear wherein the effect of noise currents induced by switchgear operating and control elements is minimized.